Hey,
ich suche ein Lied,
das kommt bei romantischen Stellen in Filmen oft, zB bei King of Queens wenn Deekan mit seiner Frau schläft oder auch bei Austin Powers 2 wenn Dr. Evil mit seiner Sekretärin da schläft, kann mir da einer helfen?
Also bei Austin kommts auf jedenfall, bin mir nicht 100pro sicher dass das bei KoQ das gleiche ist....
Network Working Group P. Mockapetris
Request for Comments: 1034 ISI
Obsoletes: RFCs 882, 883, 973 November 1987
DOMAIN NAMES - CONCEPTS AND FACILITIES
1. STATUS OF THIS MEMO
This RFC is an introduction to the Domain Name System (DNS), and omits
many details which can be found in a companion RFC, "Domain Names -
Implementation and Specification" [RFC-1035]. That RFC assumes that the
reader is familiar with the concepts discussed in this memo.
A subset of DNS functions and data types constitute an official
protocol. The official protocol includes standard queries and their
responses and most of the Internet class data formats (e.g., host
addresses).
However, the domain system is intentionally extensible. Researchers are
continuously proposing, implementing and experimenting with new data
types, query types, classes, functions, etc. Thus while the components
of the official protocol are expected to stay essentially unchanged and
operate as a production service, experimental behavior should always be
expected in extensions beyond the official protocol. Experimental or
obsolete features are clearly marked in these RFCs, and such information
should be used with caution.
The reader is especially cautioned not to depend on the values which
appear in examples to be current or complete, since their purpose is
primarily pedagogical. Distribution of this memo is unlimited.
2. INTRODUCTION
This RFC introduces domain style names, their use for Internet mail and
host address support, and the protocols and servers used to implement
domain name facilities.
2.1. The history of domain names
The impetus for the development of the domain system was growth in the
Internet:
- Host name to address mappings were maintained by the Network
Information Center (NIC) in a single file (HOSTS.TXT) which
was FTPed by all hosts [RFC-952, RFC-953]. The total network
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RFC 1034 Domain Concepts and Facilities November 1987
bandwidth consumed in distributing a new version by this
scheme is proportional to the square of the number of hosts in
the network, and even when multiple levels of FTP are used,
the outgoing FTP load on the NIC host is considerable.
Explosive growth in the number of hosts didn't bode well for
the future.
- The network population was also changing in character. The
timeshared hosts that made up the original ARPANET were being
replaced with local networks of workstations. Local
organizations were administering their own names and
addresses, but had to wait for the NIC to change HOSTS.TXT to
make changes visible to the Internet at large. Organizations
also wanted some local structure on the name space.
- The applications on the Internet were getting more
sophisticated and creating a need for general purpose name
service.
The result was several ideas about name spaces and their management
[IEN-116, RFC-799, RFC-819, RFC-830]. The proposals varied, but a
common thread was the idea of a hierarchical name space, with the
hierarchy roughly corresponding to organizational structure, and names
using "." as the character to mark the boundary between hierarchy
levels. A design using a distributed database and generalized resources
was described in [RFC-882, RFC-883]. Based on experience with several
implementations, the system evolved into the scheme described in this
memo.
The terms "domain" or "domain name" are used in many contexts beyond the
DNS described here. Very often, the term domain name is used to refer
to a name with structure indicated by dots, but no relation to the DNS.
This is particularly true in mail addressing [Quarterman 86].
2.2. DNS design goals
The design goals of the DNS influence its structure. They are:
- The primary goal is a consistent name space which will be used
for referring to resources. In order to avoid the problems
caused by ad hoc encodings, names should not be required to
contain network identifiers, addresses, routes, or similar
information as part of the name.
- The sheer size of the database and frequency of updates
suggest that it must be maintained in a distributed manner,
with local caching to improve performance. Approaches that
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attempt to collect a consistent copy of the entire database
will become more and more expensive and difficult, and hence
should be avoided. The same principle holds for the structure
of the name space, and in particular mechanisms for creating
and deleting names; these should also be distributed.
- Where there tradeoffs between the cost of acquiring data, the
speed of updates, and the accuracy of caches, the source of
the data should control the tradeoff.
- The costs of implementing such a facility dictate that it be
generally useful, and not restricted to a single application.
We should be able to use names to retrieve host addresses,
mailbox data, and other as yet undetermined information. All
data associated with a name is tagged with a type, and queries
can be limited to a single type.
- Because we want the name space to be useful in dissimilar
networks and applications, we provide the ability to use the
same name space with different protocol families or
management. For example, host address formats differ between
protocols, though all protocols have the notion of address.
The DNS tags all data with a class as well as the type, so
that we can allow parallel use of different formats for data
of type address.
- We want name server transactions to be independent of the
communications system that carries them. Some systems may
wish to use datagrams for queries and responses, and only
establish virtual circuits for transactions that need the
reliability (e.g., database updates, long transactions); other
systems will use virtual circuits exclusively.
- The system should be useful across a wide spectrum of host
capabilities. Both personal computers and large timeshared
hosts should be able to use the system, though perhaps in
different ways.
2.3. Assumptions about usage
The organization of the domain system derives from some assumptions
about the needs and usage patterns of its user community and is designed
to avoid many of the the complicated problems found in general purpose
database systems.
The assumptions are:
- The size of the total database will initially be proportional
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to the number of hosts using the system, but will eventually
grow to be proportional to the number of users on those hosts
as mailboxes and other information are added to the domain
system.
- Most of the data in the system will change very slowly (e.g.,
mailbox bindings, host addresses), but that the system should
be able to deal with subsets that change more rapidly (on the
order of seconds or minutes).
- The administrative boundaries used to distribute
responsibility for the database will usually correspond to
organizations that have one or more hosts. Each organization
that has responsibility for a particular set of domains will
provide redundant name servers, either on the organization's
own hosts or other hosts that the organization arranges to
use.
- Clients of the domain system should be able to identify
trusted name servers they prefer to use before accepting
referrals to name servers outside of this "trusted" set.
- Access to information is more critical than instantaneous
updates or guarantees of consistency. Hence the update
process allows updates to percolate out through the users of
the domain system rather than guaranteeing that all copies are
simultaneously updated. When updates are unavailable due to
network or host failure, the usual course is to believe old
information while continuing efforts to update it. The
general model is that copies are distributed with timeouts for
refreshing. The distributor sets the timeout value and the
recipient of the distribution is responsible for performing
the refresh. In special situations, very short intervals can
be specified, or the owner can prohibit copies.
- In any system that has a distributed database, a particular
name server may be presented with a query that can only be
answered by some other server. The two general approaches to
dealing with this problem are "recursive", in which the first
server pursues the query for the client at another server, and
"iterative", in which the server refers the client to another
server and lets the client pursue the query. Both approaches
have advantages and disadvantages, but the iterative approach
is preferred for the datagram style of access. The domain
system requires implementation of the iterative approach, but
allows the recursive approach as an option.
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The domain system assumes that all data originates in master files
scattered through the hosts that use the domain system. These master
files are updated by local system administrators. Master files are text
files that are read by a local name server, and hence become available
through the name servers to users of the domain system. The user
programs access name servers through standard programs called resolvers.
The standard format of master files allows them to be exchanged between
hosts (via FTP, mail, or some other mechanism); this facility is useful
when an organization wants a domain, but doesn't want to support a name
server. The organization can maintain the master files locally using a
text editor, transfer them to a foreign host which runs a name server,
and then arrange with the system administrator of the name server to get
the files loaded.
Each host's name servers and resolvers are configured by a local system
administrator [RFC-1033]. For a name server, this configuration data
includes the identity of local master files and instructions on which
non-local master files are to be loaded from foreign servers. The name
server uses the master files or copies to load its zones. For
resolvers, the configuration data identifies the name servers which
should be the primary sources of information.
The domain system defines procedures for accessing the data and for
referrals to other name servers. The domain system also defines
procedures for caching retrieved data and for periodic refreshing of
data defined by the system administrator.
The system administrators provide:
- The definition of zone boundaries.
- Master files of data.
- Updates to master files.
- Statements of the refresh policies desired.
The domain system provides:
- Standard formats for resource data.
- Standard methods for querying the database.
- Standard methods for name servers to refresh local data from
foreign name servers.
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2.4. Elements of the DNS
The DNS has three major components:
- The DOMAIN NAME SPACE and RESOURCE RECORDS, which are
specifications for a tree structured name space and data
associated with the names. Conceptually, each node and leaf
of the domain name space tree names a set of information, and
query operations are attempts to extract specific types of
information from a particular set. A query names the domain
name of interest and describes the type of resource
information that is desired. For example, the Internet
uses some of its domain names to identify hosts; queries for
address resources return Internet host addresses.
- NAME SERVERS are server programs which hold information about
the domain tree's structure and set information. A name
server may cache structure or set information about any part
of the domain tree, but in general a particular name server
has complete information about a subset of the domain space,
and pointers to other name servers that can be used to lead to
information from any part of the domain tree. Name servers
know the parts of the domain tree for which they have complete
information; a name server is said to be an AUTHORITY for
these parts of the name space. Authoritative information is
organized into units called ZONEs, and these zones can be
automatically distributed to the name servers which provide
redundant service for the data in a zone.
- RESOLVERS are programs that extract information from name
servers in response to client requests. Resolvers must be
able to access at least one name server and use that name
server's information to answer a query directly, or pursue the
query using referrals to other name servers. A resolver will
typically be a system routine that is directly accessible to
user programs; hence no protocol is necessary between the
resolver and the user program.
These three components roughly correspond to the three layers or views
of the domain system:
- From the user's point of view, the domain system is accessed
through a simple procedure or OS call to a local resolver.
The domain space consists of a single tree and the user can
request information from any section of the tree.
- From the resolver's point of view, the domain system is
composed of an unknown number of name servers. Each name
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RFC 1034 Domain Concepts and Facilities November 1987
server has one or more pieces of the whole domain tree's data,
but the resolver views each of these databases as essentially
static.
- From a name server's point of view, the domain system consists
of separate sets of local information called zones. The name
server has local copies of some of the zones. The name server
must periodically refresh its zones from master copies in
local files or foreign name servers. The name server must
concurrently process queries that arrive from resolvers.
In the interests of performance, implementations may couple these
functions. For example, a resolver on the same machine as a name server
might share a database consisting of the the zones managed by the name
server and the cache managed by the resolver.
3. DOMAIN NAME SPACE and RESOURCE RECORDS
3.1. Name space specifications and terminology
The domain name space is a tree structure. Each node and leaf on the
tree corresponds to a resource set (which may be empty). The domain
system makes no distinctions between the uses of the interior nodes and
leaves, and this memo uses the term "node" to refer to both.
Each node has a label, which is zero to 63 octets in length. Brother
nodes may not have the same label, although the same label can be used
for nodes which are not brothers. One label is reserved, and that is
the null (i.e., zero length) label used for the root.
The domain name of a node is the list of the labels on the path from the
node to the root of the tree. By convention, the labels that compose a
domain name are printed or read left to right, from the most specific
(lowest, farthest from the root) to the least specific (highest, closest
to the root).
Internally, programs that manipulate domain names should represent them
as sequences of labels, where each label is a length octet followed by
an octet string. Because all domain names end at the root, which has a
null string for a label, these internal representations can use a length
byte of zero to terminate a domain name.
By convention, domain names can be stored with arbitrary case, but
domain name comparisons for all present domain functions are done in a
case-insensitive manner, assuming an ASCII character set, and a high
order zero bit. This means that you are free to create a node with
label "A" or a node with label "a", but not both as brothers; you could
refer to either using "a" or "A". When you receive a domain name or
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RFC 1034 Domain Concepts and Facilities November 1987
label, you should preserve its case. The rationale for this choice is
that we may someday need to add full binary domain names for new
services; existing services would not be changed.
When a user needs to type a domain name, the length of each label is
omitted and the labels are separated by dots ("."). Since a complete
domain name ends with the root label, this leads to a printed form which
ends in a dot. We use this property to distinguish between:
- a character string which represents a complete domain name
(often called "absolute"). For example, "poneria.ISI.EDU."
- a character string that represents the starting labels of a
domain name which is incomplete, and should be completed by
local software using knowledge of the local domain (often
called "relative"). For example, "poneria" used in the
ISI.EDU domain.
Relative names are either taken relative to a well known origin, or to a
list of domains used as a search list. Relative names appear mostly at
the user interface, where their interpretation varies from
implementation to implementation, and in master files, where they are
relative to a single origin domain name. The most common interpretation
uses the root "." as either the single origin or as one of the members
of the search list, so a multi-label relative name is often one where
the trailing dot has been omitted to save typing.
To simplify implementations, the total number of octets that represent a
domain name (i.e., the sum of all label octets and label lengths) is
limited to 255.
A domain is identified by a domain name, and consists of that part of
the domain name space that is at or below the domain name which
specifies the domain. A domain is a subdomain of another domain if it
is contained within that domain. This relationship can be tested by
seeing if the subdomain's name ends with the containing domain's name.
For example, A.B.C.D is a subdomain of B.C.D, C.D, D, and " ".
3.2. Administrative guidelines on use
As a matter of policy, the DNS technical specifications do not mandate a
particular tree structure or rules for selecting labels; its goal is to
be as general as possible, so that it can be used to build arbitrary
applications. In particular, the system was designed so that the name
space did not have to be organized along the lines of network
boundaries, name servers, etc. The rationale for this is not that the
name space should have no implied semantics, but rather that the choice
of implied semantics should be left open to be used for the problem at
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RFC 1034 Domain Concepts and Facilities November 1987
hand, and that different parts of the tree can have different implied
semantics. For example, the IN-ADDR.ARPA domain is organized and
distributed by network and host address because its role is to translate
from network or host numbers to names; NetBIOS domains [RFC-1001, RFC-
1002] are flat because that is appropriate for that application.
However, there are some guidelines that apply to the "normal" parts of
the name space used for hosts, mailboxes, etc., that will make the name
space more uniform, provide for growth, and minimize problems as
software is converted from the older host table. The political
decisions about the top levels of the tree originated in RFC-920.
Current policy for the top levels is discussed in [RFC-1032]. MILNET
conversion issues are covered in [RFC-1031].
Lower domains which will eventually be broken into multiple zones should
provide branching at the top of the domain so that the eventual
decomposition can be done without renaming. Node labels which use
special characters, leading digits, etc., are likely to break older
software which depends on more restrictive choices.
3.3. Technical guidelines on use
Before the DNS can be used to hold naming information for some kind of
object, two needs must be met:
- A convention for mapping between object names and domain
names. This describes how information about an object is
accessed.
- RR types and data formats for describing the object.
These rules can be quite simple or fairly complex. Very often, the
designer must take into account existing formats and plan for upward
compatibility for existing usage. Multiple mappings or levels of
mapping may be required.
For hosts, the mapping depends on the existing syntax for host names
which is a subset of the usual text representation for domain names,
together with RR formats for describing host addresses, etc. Because we
need a reliable inverse mapping from address to host name, a special
mapping for addresses into the IN-ADDR.ARPA domain is also defined.
For mailboxes, the mapping is slightly more complex. The usual mail
address @ is mapped into a domain name by
converting into a single label (regardles of dots it
contains), converting into a domain name using the usual
text format for domain names (dots denote label breaks), and
concatenating the two to form a single domain name. Thus the mailbox
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RFC 1034 Domain Concepts and Facilities November 1987
HOSTMASTER@SRI-NIC.ARPA is represented as a domain name by
HOSTMASTER.SRI-NIC.ARPA. An appreciation for the reasons behind this
design also must take into account the scheme for mail exchanges [RFC-
974].
The typical user is not concerned with defining these rules, but should
understand that they usually are the result of numerous compromises
between desires for upward compatibility with old usage, interactions
between different object definitions, and the inevitable urge to add new
features when defining the rules. The way the DNS is used to support
some object is often more crucial than the restrictions inherent in the
DNS.
3.4. Example name space
The following figure shows a part of the current domain name space, and
is used in many examples in this RFC. Note that the tree is a very
small subset of the actual name space.
In this example, the root domain has three immediate subdomains: MIL,
EDU, and ARPA. The LCS.MIT.EDU domain has one immediate subdomain named
XX.LCS.MIT.EDU. All of the leaves are also domains.
3.5. Preferred name syntax
The DNS specifications attempt to be as general as possible in the rules
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RFC 1034 Domain Concepts and Facilities November 1987
for constructing domain names. The idea is that the name of any
existing object can be expressed as a domain name with minimal changes.
However, when assigning a domain name for an object, the prudent user
will select a name which satisfies both the rules of the domain system
and any existing rules for the object, whether these rules are published
or implied by existing programs.
For example, when naming a mail domain, the user should satisfy both the
rules of this memo and those in RFC-822. When creating a new host name,
the old rules for HOSTS.TXT should be followed. This avoids problems
when old software is converted to use domain names.
The following syntax will result in fewer problems with many
applications that use domain names (e.g., mail, TELNET).
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